1. Field of the Invention
The present invention relates to a switching power supply such as a ringing choke converter (RCC) type switching power supply or the like which takes out a current from a secondary winding when a switching circuit for switching a current flowing through a primary winding is turned off, and more particularly relates to a switching power supply which outputs a reduction signal representing the reduction of an input voltage of a primary side (a primary side input voltage) when the primary side input voltage reduces.
2. Description of the Related Art
In the RCC type switching power supply, when the voltage of the primary side DC power supply obtained by rectifying and smoothing the voltage of the commercial power supply system decreases, the operation area where the switching transistor is turned on is not in a completely saturated area. As a result, the collector-emitter voltage of the switching transistor increases when it is turned on. When the collector-emitter voltage of the switching transistor increases, the switching transistor may be broken due to the increase of an amount of heat generated by itself. In view of this matter, Japanese Patent Unexamined Publication No. Hei. 3-169258 proposes a technique for stopping the switching operation of the switching transistor when the voltage of the primary side DC power supply decreases.
In this technique, a series circuit formed by a resistor and a voltage regulation diode is connected between the positive terminal of a primary side power supply and the base of a primary side output transistor, whereby the series circuit formed by the resistor and the voltage regulation diode supplies a starting voltage to the primary side output transistor. According to such an arrangement, when the voltage of the primary side power supply decreases and no current flows into the voltage regulation diode, the-primary side output transistor stops oscillating. Thus, such a phenomenon is prevented from being occurred that the primary side output transistor is broken due to the decrease of the voltage of the primary side power supply.
However, when the above conventional technique is applied to a switching power supply to which the DC power obtained by rectifying and smoothing the output of the commercial power supply system, there arises the following problem. That is, the voltage obtained by rectifying and smoothing the output of the commercial power supply system exceeds 100 volts (V). Thus, when the voltage of the DC power supply decreases below 60V, for example, it is required to stop the switching operation in order to prevent the breakage of the primary side output transistor. To this end, the voltage regulation diode is required to be one having a Zener voltage of 60V. However, since the Zener voltage of the voltage regulation diode is 30V or less in general, it is required to connect two voltage regulation diodes in series in order to obtain a voltage regulation diode having a Zener voltage of 60V, whereby the cost of the parts becomes expensive.